Battery-electric vehicles (BEV) and hybrid-electric vehicles (HEV) include one or more electric machines (propulsion motors) as part of a vehicle propulsion system. An HEV, in addition, includes a heat engine such as an internal combustion engine as part of the vehicle propulsion system. HEV may be generally categorized as strong or mild. Strong hybrids are characterized by high torque propulsion motors capable of imparting significant driveline torque contributions across a wide range of vehicle operating regions for traction and regenerative braking. Strong hybrids may further take form in series hybrid architecture, parallel hybrid architecture and series/parallel architectures. Mild hybrids are generally implemented in parallel hybrid architectures. Mild hybrids are characterized by lower torque propulsion motors capable of more limited driveline torque contributions in terms of magnitude and vehicle operating regions.
One desirable feature of HEV is engine off idling, which allows for the internal combustion engine to be shut down during decelerations and traffic signal stops, for example. Such engine off idling can contribute to improvements in engine emissions and fuel economy. Engine starting and electric power generation may be accomplished via the same electric machines used for propulsion and regenerative braking thereby supplanting the need for conventional starter motor and alternator. Engine off idling requires an electrically powered accessory drive system implemented, for example, by individual direct motor driven pumps, compressors or other rotating machines, or an electrically driven accessory belt and pulley system for imparting rotating motion to a plurality of such rotating machines.
HEV include high and low voltage electrical systems. The high-voltage electrical system supports the energy and power requirements of the electric traction machines. The low voltage system supports vehicle electrical accessories. High-voltage and low-voltage electrical systems are interfaced via DC-DC converters. Electrical energy storage is typically via one or more energy storage devices such as batteries. Batteries may be associated with either the high-voltage or the low voltage electrical systems or both.
Generally, as between power density and energy density, power density is a more critical characteristic for batteries servicing HEV electric machines. Hence, the types of batteries employed in HEV are designed to sustain high-currents. Consequently, deep discharges of batteries are not generally desirable as such tend to shorten useful battery life, and battery state of charge (SOC) is desirably maintained in a relatively narrow range toward an upper region thereof. However, vehicle electrical accessory loads during engine off idle can be a substantial source of battery discharge. An additional source of significant battery discharge during engine off idle is any electrical motor associated with operation of an electrically powered accessory drive system. It can therefore be appreciated that the electrical accessory and propulsion motor electrical power density and energy density requirements are divergent. Employing a high power density battery in HEV will suffer battery longevity. Similarly, employing a high energy density battery will suffer HEV performance and operational flexibility.
While it is generally well recognized that high power electrical storage devices, such as supercapacitors, may provide many of the desired characteristics in a high voltage energy source for HEV, it is also well recognized that supercapacitors store far less energy than batteries and can be depleted rapidly when subjected to sustained electrical loads, which is an undesirable trait for vehicular use since periods of inactivity would result in unacceptable starting voltage states or accelerated discharge of vehicle batteries during such periods.